Driven by the urgent requirements of the oil and gas industry, subsea pressure boosting is a technology subject to extensive effort for further development. Due to the size, power and flow to be handled, and special requirements for operation subsea, solutions found viable for pumps for other applications may be useless subsea for the intended purpose.
For equipment subsea, reliability is usually the main issue of concern, due to huge technical, economic and environmental effects if the equipment fails.
Factors contributing to failure for new and existing subsea pump design concepts, include inter alia mechanical instability at the size and pressures required; too large pressure impacts at start, stop or abrupt load changes; breakdown of electrical insulation before expiry of design service life; accumulation of contaminations in the motor or bearings, and loss of control for several other reasons.
A promising design for improving subsea pumps is to implement a magnetic coupling between the electric motor and pump, by arranging a diaphragm or separation wall sealingly between the motor and pump, having the magnetic coupling through the diaphragm or wall. This is called a sealed pump design, since the motor is arranged in a sealed compartment.
For subsea pumps of the above mentioned type, this has however been more difficult than expected in practice, for several reasons.
Reference is made to the prior art patent publication GB 2 390 750 B, assumed to be the closest art to the present invention. Said publication describes and illustrates an electric submersible pumping (ESP) system, which shall be arranged in a wellbore for lifting the fluid collected in the well. The pump of GB 2 390 750 B has a sealed, oil filled motor housing, the motor housing is coupled magnetically through a sealing cylindrical wall to a pump, the dynamic stability of the magnetic coupling being enhanced by at least two radially spaced intermediate bearings respectively disposed inside and outside said cylindrical housing. Further, pressure balancing means maintain the pressure within the sealed housing to be substantially equal to the pressure in the wellbore. The ESP of GB 2 390 750 B must be very long in order to be feasible for operation in wellbores whilst still boosting the pressure substantially since the wellbore puts severe restrictions on the diameter.
Contamination and accumulation of particles in the bearings can make the above mentioned ESP less reliable. Particles or gas may destroy the bearing lubrication. The tendency of non-ferromagnetic metals and hence metal bearings to become ferromagnetic under stress and strain, may make the magnetic coupling ineffective. The pressure compensation of GB 2 390 750 B functions between the well pressure and the motor housing, implying that the pump side with bearings is exposed to the well flow directly and thereby severe contamination. In a typical well, the flowing well pressure may be from some 10th's to some 100th's of bar whilst the shut in pressure may be several 100th's bar higher, all of which must be handled by the pressure compensation system, and the resulting wall thicknesses or design pressure must be adapted accordingly since the pressure compensation system cannot be expected to work perfectly or instantaneously. A thick wall construction will be less efficient with respect to magnetic coupling.
A high relative speed will exist between the sealing wall and the outer and inner rotating member. This relative speed will result in hydrodynamically generated friction heat of substantial magnitude. No cooling or means for heat removal are described in GB 2 390 750 B. Other relevant art is found in the patent publications U.S. Pat. No. 6,379,127 B1, US 2011/0274565 A1 and WO 2012/125041 A1.
The main objective of the present invention is to provide a subsea pump that is more reliable than prior art subsea pumps, for operation as mentioned above.